This invention relates to integrated circuit memories and more specifically to a content addressable integrated circuit memory.
Typically, a single power supply voltage has been used to power all circuitry of a content addressable memory (CAM) cell, both for powering CAM storage elements and the CAM circuitry for searching and indicating the presence of matching data within a CAM cell. CAMs consume much power for search operations since searches are typically conducted over a large memory space, and search data input and matchline signals are frequently driven or cycled between voltages.
The need for ever greater computing speed and integration density requires reductions in power consumption, as is the longstanding need to provide greater function in small, e.g. handheld computing equipment. U.S. Pat. No. 6,101,115 describes a system in which power consumption is reduced by providing a independent power supply at a lowered voltage for precharging the matchline. However, reducing the matchline precharge power supply alone does not adequately address the needed power reductions.
Accordingly, the present invention provides for greater power reductions than heretofore obtained in a CAM by providing search signals at input voltages that are substantially independent from an operating voltage of storage elements of the CAM .Thus, the search line power supply voltage is preferably at a lower voltage than the power supply voltage at which data is stored within storage elements of the CAM. Lowering the search line power supply voltage tends to conserve power. At the same time, maintaining a relatively high power supply voltage to the CAM storage elements tends to preserve good signal margins, data retention and low soft error rates.
Preferably, a search input voltage HIGH is selected which lies within about 0.2V above a threshold voltage of a search input device of the CAM cell. Preferably, the search input device has a lowered threshold voltage as compared to the threshold voltage of other devices used in the CAM cell. In this way, further reductions in search line power consumption are made possible.
In an aspect of the invention, a method is provided for operating a content addressable memory (CAM) within an integrated circuit. The method includes inputting search signals to the. CAM cells of the CAM at a search input voltage HIGH which is substantially independent of an operating voltage of storage elements within the CAM cells. A match signal is output upon detecting a matching condition between the search signals and data stored in the storage elements.
In another aspect of the invention, an integrated circuit including a content addressable memory (CAM) is provided, wherein the CAM includes content addressable memory (CAM) cells, each having one or more storage elements operated at an operating voltage of the CAM and the CAM cells include search input devices adapted to receive search input voltages selected substantially independently from the operating voltage.